1. Field of the Invention
The present invention relates to a spark plug and to a method of manufacturing the spark plug.
2. Description of Related Art
A spark plug used for providing ignition in an internal combustion engine such as an automobile engine typically includes a metallic shell; an insulator formed from, for example, an alumina-based ceramic and disposed within the metallic shell; and a center electrode disposed within the insulator. A ground electrode is attached to the metallic shell. The insulator axially projects from the rear opening portion of the metallic shell. A terminal metal piece is disposed inside the projecting portion of the insulator. The terminal metal piece is connected to the center electrode via a resistor, conductive glass seal layers formed in a glass sealing step and the like. Upon application of a high voltage via the terminal metal piece, spark discharge is induced at a gap formed between the ground electrode and the center electrode.
However, if the spark plug is subjected to certain conditions simultaneously, such as high plug temperature, high ambient humidity, and the like, application of high voltage cannot successfully induce spark discharge at the gap, and there may occur a so-called "flashover" phenomenon in which spark discharge is induced between the terminal metal piece and the metallic shell such that the spark runs over the surface of the projecting portion of the insulator. Therefore, in order to prevent the flashover phenomenon, most spark plugs for general use have a glaze layer formed on the surface of their insulators. The glaze layer also serves for smoothing the surface of the insulator so as to prevent contamination, and for increasing chemical or mechanical strength.
The glaze layer is formed on an insulator, through application of glaze slurry onto the surface of an insulator and firing (particularly called "glost firing"). In the case of an alumina-based insulator for use in a spark plug, a glaze layer is formed on a fired insulator through subsequent baking at 1000.degree. to 1100.degree. C. In such a case, there has conventionally been used lead-silicate-glass-based glaze which has a lowered softening point due to incorporation of a relatively large amount of PbO to silicate glass. However, this type of glaze involves the following drawbacks:
(1) Since the glaze has a coefficient of linear expansion lower than that of alumina-based insulating material which serves as a substrate, the obtained glaze layer is susceptible to cracks and the like.
(2) Although the glaze contains a considerable amount of PbO, the glost-firing temperature is still high; i.e., 1000.degree. C. or more. In the manufacture of spark plugs, glost firing is often performed concurrently with a glass sealing step so as to reduce the number of manufacturing steps. Therefore, high glost-firing temperature as described above disadvantageously permits accelerated oxidation of a terminal metal piece and a center electrode. A conceivable measure for further lowering the glost-firing temperature is to add alkali metal oxide such as Na.sub.2 O to the glaze. However, if the amount of alkali metal components is increased excessively, insulation performance is lowered, and the spark plug becomes susceptible to flashover.
(3) In recent years, concerns for environmental protection have increased worldwide, and glaze containing Pb has been used less often. For example, the automobile industry, which uses a large number of spark plugs, is considering the complete abolition of spark plugs using glaze containing Pb, in view of the environmental effect of discarded spark plugs.